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March 24th 04, 11:59 AM

On 23 Mar 2004 23:16:23 -0800, (Tom Bruhns) wrote:

Paul,

It's a series resonant circuit. The inductor is obvious. The
capacitor is actually split between the one to ground at the gate
input and the one on the other side of the inductor. Since it's a
series circuit, the inductor and first cap can be swapped, of course;
that might make it more obvious. The gate's RF input impedance
(including a resistive part) and the bias resistors are in parallel
(AC-wise) with the cap to ground, and provide a certain amount of
damping -- lowering of the Q -- or the reason loaded Q is lower than
Qu of the inductor. Since the tuning cap is in series with that cap,
only part of the resonance voltage appears across it. Remember, in a
series-resonant circuit, the voltage across the inductor or capacitor
is much higher than the exciting voltage--that's where the voltage
step-up comes from that's needed in this case. Splitting the net
capacitance up this way lets you get a reasonably high loaded Q (to
reject the other harmonics better) and control the output voltage, and
provide the proper load at to the driving gate...as I recall, my
design goal was about 100 ohms at the fifth harmonic, and a much
higher impedance at the fundamental and the other harmonics. That
way, the driving gate doesn't dissipate much power trying to drive
those other harmonics into a heavy load, and only has to deliver
significant power at the fifth harmonic. -- You could just use a cap
from the gate input to ground, and an inductor to the driving gate's
output (and then do away with the bias resistors too...), but then you
can't so easily control the loaded Q.

Thanks for the clarification, Tom. A useful piece of enlightenment.
Most helpful.

p.
--

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